The present invention relates to plural, color image exposures formed on a photoreceptor belt by at least one LED print bar and, more particularly, to improvements in image exposure through previously exposed and developed images.
Image print bars used in xerographic recording systems are well known in the art. The print bar generally consists of a linear array of a plurality of discrete light emitting sources. Light emitting diode (LED) arrays are preferred for many recording applications. In order to achieve high resolution, a large number of light emitting diodes, or pixels, are arranged in a linear array on a common substrate. Each LED in the linear array is used to expose a corresponding area on a moving photoreceptor to an exposure value defined by the video data information applied to the drive circuits of the print bars. The photoreceptor is advanced in the process direction to provide a desired image by the formation of sequential scan lines.
Color digital LED printers may operate in either a single pass or multiple pass mode. In a single pass, process color system, three imagers are positioned adjacent to a moving photoreceptor surface and are selectively energized to create successive image exposures, one for each of the three process colors, cyan, magenta an yellow. A fourth black imager is usually added. A color digital printer may also operate in a highlight color mode wherein one or two colors and black are exposed. In a multiple pass system, each image area on the photoreceptor surface must make at least three revolutions (passes) relative to the transverse scan lines formed by the modulated beam generated by the imagers.
In one prior art embodiment of a single pass color xerographic printer shown in FIG. 1, a plurality of LED print bars are positioned adjacent to a photoreceptor belt surface and selectively energized to create successive image exposures. For full color, four bars are used, one for each of the three basic colors (cyan, magenta and yellow) and a fourth print bar for black images. FIG. 1 shows a color printing system having four exposure stations 10, 12, 14, 16, each station including an LED print bar 10A, 12A, 14A, 16A. Each print bar is selectively addressed by video image signals processed through Electronic Sub System (ESS) 15 and controlled by drive circuit 14 to produce a modulated output from each print bar which is coupled through a gradient index lens array 10B, 12B, 14B, 16B, onto the surface of previously charged photoreceptor belt 17. The length of belt 17 is designed to accept an integral number of full page image areas. Upstream of each exposure station are charge devices 18, 19, 20, 21, which place a predetermined electrical charge on the surface of belt 17. As the belt moves in the indicated direction, each image area moves past each of the print bars, with each bar providing an exposure pattern in response to the video data input. The exposure pattern begins when the leading edge of an image area reaches a transverse start-of-exposure line. The exposure pattern is formed of a plurality of closely spaced transverse scan lines. Downstream from each exposure station, a development system 26, 27, 28, 29, develops a latent image of the last exposure without disturbing previously developed images. A fully developed color image is then transferred at a transfer station 33 to an output sheet. Further details of xerographic stations in a multiple exposure single pass system are disclosed in U.S. Pat. Nos. 4,661,901; 4,791,452 and 4,833,503, whose contents are hereby incorporated by reference. It is understood that the image exposures formed by print bars 12A, 14A, 16A must be formed through toner deposited by developer stations 26, 27, 28.
A problem in prior art systems where sequential exposures are made through previously developed images is that some light energy is absorbed in exposing through previously applied toner material. Data in a color printer is applied in plates or color separations. Each color separation is imaged and developed in sequence and the second image station must image through areas of toner where the first image was developed. Thus, for example, if exposure station 10 in FIG. 1 formed a first color image (cyan) which was developed with a cyan toner at development station 26, portions of the exposure of the second (magenta) image at exposure station 12 which overlap the cyan image, would have to be made through the previously laid down magenta toner. Continuing, a third color image (yellow) printed at station 14 would have to be made through the cyan and magenta toner, to the extent that portions of the yellow image would overlap the previously formed cyan and magenta images. Since the prior art binary print bars can only write a line of data with the on pixels at one exposure, successive exposures would have more light from the print bar absorbed by the previous toner images, reducing the amount of light penetrating to the recharged surface of the photoreceptor and hence reducing the efficiency and faithfulness of the color rendition. According to a first aspect of the invention, circuitry is provided for the second and subsequent exposure stations which would generate signals to the print bars of the second and subsequent print bars to increase exposure to these print bars so as to compensate for the absorption of light due to the previous exposure(s). Thus, in a modification to the FIG. 1 embodiment, color plate information input to print bar 10A would also applied to print bar 12A and 14A in such a way as to increase the output energy needed to efficiency image through the previously applied toner. More particularly, the present invention relates to a color printer for forming successive color latent images on the surface of a photoreceptor belt, moving in a process direction, each latent image subsequently developed with a color toner with successive image exposures accomplished in superimposed registration with previously developed color images, the printer including:
means for charging the photoreceptor belt surface,
print bar means for producing a plurality of exposure patterns on the belt in accordance with binary image data signals representing a plurality of colors, said exposure patterns formed by addressing selective pixels in said print bar means during a pixel time interval defining a color image line,
means for developing each exposure pattern with a color toner,
means for recharging each developed image, said print bar means being successively addressed to form subsequent color exposure patterns by exposing the recharged photoreceptor through said previously developed color toner, and
image data processing circuit means for recognizing that at least a portion of a subsequent color exposure pattern is to be formed through a previously developed color toner and including means for increasing exposure in at least said portion.